<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
<script type="text/javascript">
function isnum(){
var v="start"
document.getElementById("demo").innerHTML=v;
var a=document.getElementById("tel").value;
if(a==''||isNaN(a)){
alter("不是数字");
}else{
		
		}
}
</script>
</head>
<body>
<p id="demo"></p>
<script type="text/javascript">
	var v="hello";
	document.getElementById("demo").innerHTML=v;
</script>

<input id="tel" type="text"/>
<button type="button" onclick="isnum()">点击我</button>
</body>
</html>

